The present invention relates to gamma cameras, and particularly to the detector head construction of such cameras.
The gamma camera is increasingly being used as a medical diagnostic instrument for the location and display of abnormalities in human glands and organs. It provides an image representing the distribution of a gamma-ray-emitting tracer material administered to the patient, which material is selected so as to be more greatly absorbed than the other tissues by the abnormality to be detected. The detector head of the gamma camera includes a scintillation crystal which receives the gamma rays emitted by the patient and converts them to light photons, a collimator for transferring the gamma-ray image from the patient to one face of the scintillation crystal, and a plurality of photomutiplier tubes arrayed at the opposite face of the crystal for receiving the light photons emitted at the scintillation points of the crystal as a result of the absorption thereby of the gamma rays. The photomultiplier tubes output pulses which contain information on both the energy absorbed from the gamma rays impinging on the crystal, and also the position of the scintillation points or events within the crystal. These pulses are fed to an arithmetic circuit which produces signals for display on a cathode ray tube, or for further processing by an image processor.
Efforts are continously being made to increase the camera sensitivity, which depends upon its detection efficiency, and also to increase the camera spatial resolution, which refers to its ability to distinguish in the display system between each of two close, isolated, scintillating points. Increasing the thickness of the scintillation crystal increases its detection efficiency, but reduces the spatial resolution of positional accuracy of each scintillation point because of the larger area from which the photons are accepted by each photomultiplier tube. The typical gamma camera now in use includes a scintillation crystal of thallium activated sodium iodide, NaI(TI), of approximately 13 mm thickness, which crystal has an index of refraction (.mu..sub.SC) of 1.84. The conventional gamma camera also includes a light guide acting as an optical coupling between the exit field of the scintillation crystal and the photomultiplier tubes, the light guide having a index of refraction (.mu..sub.LG) of about 1.150 in the conventional gamma camera.